Biofuels

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There’s an interesting little piece in South African Airlines’ in-flight magazine this month, describing how the airline is partnering with Boeing and a company called SkyNRG to make a renewable jet fuel from the Tobacco plant. ‘Put this in your pipe – and fly it!’ they say, claiming a potential 50–80% reduction in carbon footprint.

(We need to talk about the role of stratospheric water vapour in climate change, sometime, but meanwhile…)

The use of tobacco is an interesting way to sidestep a common criticism of biofuels; that they aren’t ethical because the biofuel replaces a food crop. Smoking is unfashionable now, and demand for tobacco is falling: so you find a hybrid tobacco plant, selected for yielding a high quantity of oil-rich seeds, and growers can continue to use their existing knowledge, practices and machinery. It’s a pretty neat idea.

A better one is the creation of biofuel from an algae that grows on sewage (Virgin and Air New Zealand were both involved in that as far back as 2008) but it’s not half bad.

I read this while on board an Airbus A330-200, flying from London to Johannesburg; a journey of something over 9,000 kilometres. Time for some rough calculations… with a maximum range of 13,400km, the A330 holds up to 139,100 litres of fuel, so on the long-haul flight described, allowing for some deviations for weather, other traffic, holding patterns and so on, let’s say we get through 100,000 litres of fuel.

How many tobacco seeds do you have to press to get a litre of oil? And how much of that oil can be fractioned into jet fuel? That is the big question, here.

In their paper, ‘Comparison of techniques for the extraction of tobacco seed oil’, Stanisavljevic et al (2009) report that in tobacco up to 40% of the total seed mass is oil. The best technique for oil extraction was cold pressing, delivering 93% of the oil content. The by-product ‘cake’ is useful for cattle-feed (don’t worry, the nicotine isn’t present in the seeds). Some research (including Giannelos et al, 2002) has shown that tobacco seed oil can be used as a diesel substitute – not a bad use for it as the oil is high in cholesterol.

Having viewed the websites of enthusiasts who grow their own tobacco, I can report the following: a plant can have many flowers, and when these reach maturity they produce seed pods. The reported number varies, but around a eighty on a single plant appears to be common. Each seed pod contains a mass of seeds; hundreds or even thousands of the things… but each is smaller than a grain of sand.

This is the bit we’re after… although it’s possible the remainder of the biomass will also be of some use.

Now, if you’re really interested you can download and read ‘Transgenic Tobacco Overexpressing Brassica juncea HMG-CoA Synthase 1 Shows Increased Plant Growth, Pod Size and Seed Yield: e98264’ by Liao et al (2014). Alternatively, you could wait for the movie to come out, but I think you’ll be waiting a long time.

Liao et al (2014) compares five strains of tobacco, and the most promising for our purposes appears to yield around 1,250 seeds per pod. Its average dry weight for a hundred seeds is also among the best… at just under 0.01g.

Let’s assume that SkyNRG’s tobacco hybrid is as oil-rich as it can possibly be. You can probably squeeze 19,750 tobacco plants into each hectare of land at your disposal. Let’s assume all plants survive. Each produces eighty pods. Each pod conforms to the highest yield seen by Liao et al (2014), and we use the most efficient cold pressing technique for extraction.

The number of seeds harvested is impressive, at almost two billion. That’s almost 188 kilos of tiny seeds… from which we get 61.7 litres of oil.

Exactly what fraction of that oil is suitable for use in a gas turbine, I don’t know, although jet engines aren’t particularly fussy, so let’s be generous and say it can all be used. To generate the 100,000 litres used up in a one-way trip between London and Johannesburg is going to require 1,620 hectares’ production – in other words a land area of 16.2 square kilometres… which doesn’t seem much when considered against the massive 1,221,037 km2 land area of South Africa. We have to remember, though, that the Airbus A330 in our study doesn’t just fly once a year; more likely hundreds of times a year. Perhaps you can get two tobacco crops from each plot of land, per year… but even so, the numbers for land use are adding up.

Furthermore, we have to keep in mind that agriculture doesn’t happen ‘for free’; there’s people to pay and to feed, machinery to run and water to find, all the while (it is to be hoped) preserving the spectacular biodiversity of the nation.

Is this anything more than a puff piece for South African Airlines, establishing their environmental credentials and perhaps making the passengers feel a little bit better about their carbon footprint? I think… maybe. It’s just about viable (although commercial success would require the economic use of the rest of the plant), and it is certainly worth conducting experiments of this kind. Most oilseed crops yield something like one metric tonne per hectare, which makes my estimate for tobacco’s contribution look absolutely puny at 61.7 litres… but it makes use of existing expertise in the growing community, and suggests a way to achieve continuity at a time when demand for tobacco is falling. That’s got to count for something.

If SkyNRG are right, we’ll be seeing a lot more of this…

Of greater interest, and perhaps showing a more mainstream future for biofuels, was a conference paper that I saw presented in Malawi: the work of Mr Evans Kacelenga on the blending of locally-produced methanol in Malawi’s petrol supply (and the reasons why it’s not happening at the government’s 20% target) showed that the exploitation of biofuels is complicated: it’s a problem of politics, economics and logistics, just as much as it’s a technical challenge.